In the third-floor rock-and-mineral galleries at Yale Peabody Museum of Natural History in New Haven are some spectacular and beautiful geological wonders: a smooth, transparent pink calcite, a sea-green fluorite that looks like a big jade teddy bear, a tall, pointed purple quartz geode.

The center of attention in the gallery now, however, is a small rock, about the size of an egg, craggy, in gray, green and a speckling of orange. It spins on a tiny stand, protected by thick glass. Seen on the surface, it's oddly colored but otherwise looks like just another rock.

But what stories it might tell if it could talk.

"It's unique. It does not have a counterpart in any rock we know about," said University of Washington researcher Anthony Irving. "... It's an enigma, a mystery."

Scientists believe the little rock — which weighs about a quarter pound and was found in 2012 in the Sahara Desert in Morocco — is the first-ever piece of the planet Mercury found on Earth.

Of course, since no rock samples ever have been taken from Mercury, there is still room for doubt. But the evidence in favor of the little meteorite is strong.

"The Messenger [spacecraft] measured aspects of the surface of Mercury, measured oxygen isotopes," Irving said. "[The rock] is rich in chromium and low in iron. ... It's one of the least magnetized rocks ever measured. It's tantalizing, the things we're accumulating about it."

The meteorite is the largest fragment of a meteorite known as NWA 7325 that was found by nomadic tribesmen in 2012 as a scattering of 35 green stones. Most of the other stones were mishandled before being acquired by scientists, so getting information out of the other fragments would be difficult.

Irving, an expert in extraterrestrial rocks, said when the rock was first pointed out, he knew it was different. "It's not the moon. It's not right. It's not Mars, Earth or Venus. Venus is too strong. It's hard to get things off Venus," he said, referring to that planet's gravity and atmosphere. "It didn't match anything we knew."

The green coloring is the rock's "fusion crust," and the orange speckles are from dust and microbes picked up after it landed on Earth, which could have been as far back as 10,000 years ago.

About 47,000 meteorites are known to exist on Earth. Only 67 are known to be from Mars and 177 from the Earth's moon. Many other meteorites are of unknown origin, but may have originated in the asteroid belt between Mars and Jupiter. However, the NWA 7325 is the only one believed to be from Mercury.

Irving said it is believed the rock took 24.5 million years to travel to Earth, but the distance is uncertain. "It doesn't tell you how many times around the sun it went, how many times around the Earth," he said.

Stefan Nicolescu, collections manager in the museum's mineraology department, said that desert nomads are known for finding extraterrestrial rocks. This is due to their lifestyle, traveling year-round in non-forested areas, where rocks don't get lost among vegetation. Nomadic tribesmen are known to carry around rare-earth magnets to test rocks they find.

"Rocks with a color contrast jump out at you from the desert floor," Nicolescu said. "They self-train themselves to recognize them."

Those rare-earth magnets, however, are the reason the other rocks in NWA 7325 are useless for research purposes. "Those magnets are so strong they reset the magnetic record," Irving said. "The sample we got ... they used their eyes instead of their magnets."

Irving and Nicolescu said that even though nothing has proved 100 percent that the rock is from Mercury, nothing has disproved it either. And despite strong evidence — the fusion crust, the chemical similarity to the surface of Mercury, etc — there is always the possibility that they are wrong.

"The problem is, they don't come with an address," Irving said.

Both scientists welcome any inquiry. "The debate is going on. We want to keep it alive," Nicolescu said.

And who knows, that debate might go in an entirely different direction. "Maybe it's from a really interesting place we didn't know about before," Irving said.

"FROM MERCURY TO EARTH? A METEORITE LIKE NO OTHER" will be at Yale Peabody Museum of Natural History, 170 Whitney Ave. in New Haven, until Tuesday, Sept. 2. Museum hours are Monday to Saturday 10 a.m. to 5 p.m., Sunday noon to 5 p.m. Admission is $9, $8 seniors, $5 children age 3 to 18, free children younger than 3. Admission is free to all from 2 to 5 p.m. on Thursdays. Details: peabody.yale.edu.